Electrochemical cells that have porous electrodes require an electrically conductive matrix material to facilitate transportation of electrons between the electrodes. In many applications, the conductive matrix phase also serves as a support for catalyst particles that improve the reaction kinetics. Most industrial catalysts are supported inert metal oxide materials that have a high surface area and promote catalytic activity.
Fuel cell electrocatalyst manufacturers are continually trying to increase the lifetime and performance of their electrocatalysts. Increasing the lifetime of the electrocatalysts allows them to provide a more economically competitive product and improving the performance of the catalysts means that lower precious metal loadings can be used in the catalyst layers of a Membrane Electrode Assembly (MEA). The main strategy currently used to increase performance and lifetime of the electrocatalyst is to change the catalyst synthesis method or to increase the surface area of the carbon catalyst support. However, little effort has focused on developing novel catalyst support materials.
In electrochemical applications such as Polymer Electrolyte Membrane Fuel Cells (PEMFCs) and Direct Methanol Fuel Cells (DMFCs), the typical support material used is a highly conductive, high surface area carbon. However, carbon supported electrodes that operate at voltages above ˜0.9 V in the presence of water are known to undergo a corrosion reaction as shown below:C+2H2O→CO2+4H++4e−These conditions are experienced by electrocatalyst layers in PEMFC stacks as well as in water and chlorine electrolyzers.
Previous patents including U.S. Pat. Nos. 3,616,445, 3,846,273, and 4,484,999 have asserted that valve metal oxide coatings (where valve metals are assumed to be any of titanium (Ti), niobium (Nb), zirconium (Zr), hafnium (Hf), vanadium (V), molybdenum (Mo), tungsten (W), etc.) protect precious metal (i.e. platinum, palladium, rhodium, ruthenium, and iridium) catalysts inside the coating from operative cell conditions for chlorine and caustic electrochemical cells. Such processes coated a titanium substrate with a solution of liquid precursors for the valve metals and the precious metal catalysts. The coated substrate was then heated to temperatures around 600° C. in air to oxidize the precursors for the valve metals and the precious metals.
It is desirable to create an electrode for use in fuel cells with a valve metal oxide or a mixture of such oxides that protects precious metal catalysts from the destruction seen with carbon supports. However, such an electrode would be attached to a Polymer Electrolyte Membrane (PEM) and therefore could not be heated to the high temperatures described in previous patents because the PEM would be destroyed. Furthermore, a surface are of greater than 20 m2/g is desirable because larger surface area results in less voltage needed to drive a reaction.
U.S. Pat. Nos. 4,422,917, 5,173,215, and 6,818,347 describe the synthesis of various support powders for use in electrochemical applications, but the synthesis procedures involve heating mixtures of metal oxide powders to temperatures above 1000° C. and the products of the synthesis methods yield support material with a surface area less than 5 m2/g. It is even most preferred in U.S. Pat. No. 5,173,215 that particles of the support have a BET surface area less than 0.1 m2/g due to particle stability considerations. The low surface area of the particles is a drawback for using such supports for fuel cells.
Therefore, a lower temperature method is needed to synthesize a titania catalyst support. In addition there is a need for a titania catalyst support having a surface area greater than 20 m2/g and a precious metal catalyst configured for use on such a support.